Advanced propulsion and combustion applications require materials with thermal and compositional stability in aggressive environments above 1200° C. Silicon carbide (SiC) is a particularly useful material because of its relatively high strength, low density, high thermal conductivity and oxidative resistance. Typical applications for SiC include ceramic composite reinforcement, ceramic armor, turbine components, barrier coatings, catalytic structures and heat exchangers. For some of these applications, controlled porosity, including oriented pores, within a SiC structure would enable a weight reduction (e.g., for use in armor, composites), an ability to provide desired anisotropic properties or a superior flow field for heating or cooling fluids with lower pressure drop (e.g., for use in catalytic supports, heat exchangers, reactors).
Ceramic foam containing SiC is typically created by infiltrating a ceramic-containing slurry throughout a polymer foam, followed by burnout of the polymer foam and post-densification of the slurry to form the desired reticulated ceramic foam (FIG. 1). This product contains oxide bonding between SiC particulate to form the foam structure, and does not exhibit the ideal characteristics of SiC because of the residual oxide phase required to produce such a foam. Ceramic foam products derived from SiC particulate are commercially available from Porvair PLC, Norfolk, United Kingdom, and their business Selee Corporation, Hendersonville, N.C.
Ultramet of Pacoima, Calif., produces a similar reticulated SiC foam using a chemical vapor deposition or infiltration process (CVI) over a pyrolyzed carbon foam (FIG. 2). The carbon foam skeleton is derived from the pyrolysis of a thermosetting polymer foam, and the resulting rigid vitreous carbon structure is marketed as Ultrafoam. The Ultramet SiC products are produced by the vapor infiltration and deposition of SiC directly upon the pyrolyzed carbon reticulated foam skeleton. In this process, 10 to 1000 microns of ceramic is deposited on the carbon foam at elevated temperatures suitable for the desired deposition reaction. The carbon skeleton functions only as a substrate for the material being deposited. Following deposition, the carbon substrate core is removed, leaving behind voids within the ceramic struts. Ceramic foams ranging in density from 10 to 100 pores per inch are offered by Ultramet, corresponding to a typical pore size ranging from 250 microns to 2500 microns. This vapor deposition process is commonly performed within the industry using a chlorinated or methylchlorinated silane compound such as trichloromethyl silane, Cl3(CH3)Si, or related gaseous and/or liquid precursors to SiC, often in the presence of H2 gas. The aforementioned Ultramet ‘Ultrafoam’ product provides physical properties closer to ideal SiC because of the material purity. However, when observing the microphotograph of FIG. 2, one of ordinary skill in the art will notice the relatively thick ‘struts’ of about 100 microns present in the Ultramet product. Such thick struts surround individual foam cells, which generally outline a void space having a polygonal shape. In addition, thick struts can prevent creation of a SiC structure that maintains a desirable balance of relatively high strength, lower stiffness, controlled porosity and pore geometry, high surface area and low density. Furthermore, struts that retain trapped void space and porosity can disadvantageously prevent access to functional surfaces, thereby diminishing desired properties.
Thus, there is a need for a porous SiC-containing article and a relatively low cost fabrication method suitable for creating such an article with controlled porosity.